The present invention relates to a semiconductor laser usable for pumping an Er-doped quartz optical fiber amplifier and other purposes. The invention also relates to a stripe-type semiconductor laser having a GaAs substrate.
An Er-doped quartz optical fiber amplifier now attracts much attention as being a device and technology important to the next generation of optical communication systems. A semiconductor laser having a wavelength of 1.48 .mu.m or 0.98 .mu.m is used as a pumping light source for that optical amplifier. There has been proposed a 0.98-.mu.m semiconductor laser which has a GaInAs active layer and AlGaAs or GaInP cladding layers.
In general, to increase the efficiency of introducing laser output power to an optical fiber by elongating a near-field pattern in the vertical direction, an active layer and optical confinement layers need to be made thinner. However, thinning the active layer and optical confinement layers deteriorates the optical confinement efficiency. As a result, carriers are confined insufficiently, to thereby deteriorate the temperature characteristics of the semiconductor laser. Due to the above phenomena inherent in a semiconductor laser, the above-mentioned 0.98-.mu.m semiconductor laser cannot satisfy both requirements of high output power and superior temperature characteristics which are required for a pumping light source for the Er-doped quartz optical fiber amplifier.
Turning to another subject, it is known to use a 1-.mu.m-band semiconductor laser, which uses a silicon oxide film or a silicon nitride film, to confine light and current. While this type of conventional semiconductor laser can be manufactured easily, it has a disadvantage of low heat dissipation.
In view of the above, semiconductor lasers are now being developed in which light and current are confined by a structure made of only semiconductors. In the case of semiconductor lasers in which the energy of oscillation light is greater than or equal to the band gap energy of GaAs, light can be substantially confined by forming a mesa portion (protrusion strip) in a clad and filling the side regions of the mesa portion with GaAs, where the side portions absorb guided light. Because this type of structure has not yet been employed in 1-.mu.m-band semiconductor lasers, various attempts to do so have been made with respect to the 1 .mu.m-band semiconductor lasers.
Among those attempts, a technique of filling the side regions of a clad mesa portion with a material whose effective refractive index is smaller than the clad is now being investigated actively, because this type of structure can be produced relatively easily. Two examples of this technique are discussed below. In the first example, a clad is made of AlGaAs and the side regions of a meas portion is filled with GaInP whose refractive index is smaller than AlGaAs (see Chida et al., The 40th Spring Conference of the Japan Society of Applied Physics, Presentation No. 1a-C-2 (1993)). In the second example, a mesa portion including a high-refractivity GaAs layer is formed in a clad made of GaInP, and the side regions of the GaAs layer is filled with GaInP (see Sagawa et al., The 40th Spring Conference of the Japan Society of Applied Physics, Presentation No. 31p-C-11 (1993)).
These two techniques employ the real refractive index waveguide structure, in which the side regions of a mesa portion are filled with a low-refractivity material. On the other hand, a red semiconductor laser of an index antiguiding structure has been proposed in which the side regions of a mesa portion of an AlGaInP clad are filled with AlGaInP of a smaller Al proportion (see Kidoguchi et al., The Autumn Conference of the Japan Society of Applied Physics, Presentation No. 18a-V-5 (1992)). Since this index antiguiding structure can produce a large difference between thresholds of the fundamental transverse mode and higher order transverse modes, it can readily provide a single transverse mode operation.
However, the first conventional technique of Chida et al. has a problem of a small control range of optical confinement, because the clad material is limited to AlGaAs whose refractive index is larger than GaInP as the embedding material. In the second conventional technique of Sagawa et al., the increase of the refractive index difference in the horizontal direction will necessarily be associated with excessive concentration of light in the GaAs layer.
Further, the third conventional technique of Kidoguchi et al., wherein the index antiguiding structure can readily provide a single transverse mode operation, requires the AlGaInP light diffusion layer (which buries the mesa portion (protrusion strip)) to have a small Al proportion (Ga/(Al+Ga)) of 0.6. It is difficult to control growth conditions and pre-treatment conditions of the growth of the AlGaInP light diffusion layer. That is, because the active layer of the semiconductor laser of Kidoguchi et al. is made of GaInP, whose band gap energy is much larger than GaAs, the Al proportion of the clad needs to be increased to effectively confine carriers, necessitating the increase of the Al proportion of the light diffusion layer.